Fluorenyl based metallocene catalysts are effective catalysts in the polymerization, including homopolymerization or copolymerization of olefin polymers such as ethylene, propylene and higher olefins or other ethylenically unsaturated monomers.
Fluorenyl-type metallocenes are characteristically in the form of metallocene ligand structures characterized by bridged cyclopentadienyl and fluorenyl groups. An example is isopropylidene (cyclopentadienyl)(fluorenyl) zirconium dichloride. The cyclopentadienyl group or the fluorenyl group can be modified by the inclusion of substituent groups in the cyclopentadienyl ring or the fluorenyl group which modifies the structure of the catalyst and ultimately the characteristics of the polymers produced. Thus, olefin polymers such as polyethylene, polypropylene, which may be atactic or stereospecific such as isotactic or syndiotactic, and ethylene-higher alpha olefin copolymers such as ethylene propylene copolymers, can be produced under various polymerization conditions and employing various polymerization catalysts.
The metallocene catalyst based upon a bridged cyclopentadienylfluorenyl ligand structure can be produced by the reaction of 6,6-dimethyl fulvene, which may be substituted or unsubstituted, with a fluorene, which in turn may be substituted or unsubstituted, to produce the bridged isopropylidene cyclopentadienylfluorenyl ligand structure. This ligand is, in turn, reacted with a transition metal halide such as zirconium tetrachloride to produce the bridged zirconium dichloride.
Fluorenyl ligands may be characterized by the following numbering scheme for the fluorenyl ligand as indicated in Formula (1):
In this numbering scheme, 9 indicates the bridgehead carbon atom. The remaining carbon atoms available to accept substituents are indicated by numbers 1-4, one phenyl group of the ligand, and numbers 5-8 of the other phenyl group of the fluorenyl ligand.
Alpha olefin homopolymers or copolymers may be produced using metallocene catalysts under various conditions in polymerization reactors which may be batch type reactors or continuous reactors. Continuous polymerization reactors typically take the form of loop-type reactors in which the monomer stream is continuously introduced and a polymer product is continuously withdrawn. For example, polymers such as polypropylene, polyethylene or ethylene-propylene copolymers involve the introduction of the monomer stream into the continuous loop-type reactor along with an appropriate catalyst system to produce the desired olefin homopolymer or copolymer. The resulting polymer is withdrawn from the loop-type reactor in the form of a “fluff” which is then processed to produce the polymer as a raw material in particulate form as pellets or granules. In the case of C3+ alpha olefins, such as propylene, or substituted ethylenically unsaturated monomers such as styrene or vinyl chloride, the resulting polymer product may be characterized in terms of stereoregularity, for example, isotactic polypropylene or syndiotactic polypropylene.
The structure of isotactic polypropylene can be described as one having the methyl groups attached to the tertiary carbon atoms of successive monomeric units falling on the same side of a hypothetical plane through the main chain of the polymer, e.g., the methyl groups are all above or below the plane. Using the Fischer projection formula, the stereochemical sequence of isotactic polypropylene is described as follows:
In Formula (2), each vertical segment indicates a methyl group on the same side of the polymer backbone. Another way of describing the structure is through the use of NMR. Bovey's NMR nomenclature for an isotactic pentad as shown above is . . . mmmm . . . with each “m” representing a “meso” dyad, or successive pairs of methyl groups on the same said of the plane of the polymer chain. As is known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer.
In contrast to the isotactic structure, syndiotactic propylene polymers are those in which the methyl groups attached to the tertiary carbon atoms of successive monomeric units in the chain lie on alternate sides of the plane of the polymer. Syndiotactic polypropylene using the Fisher projection formula can be indicated by racemic dyads with the syndiotactic pentad rrrr as shown by Formula (3):
In Formula (3), the vertical segments indicate methyl groups in the case of syndiotactic polypropylene, or other terminal groups, e.g. chloride, in the case of syndiotactic polyvinyl chloride, or phenyl groups in the case of syndiotactic polystyrene.
Other unsaturated hydrocarbons which can be polymerized or copolymerized with relatively short chain alpha olefins, such as ethylene and propylene include dienes, such as 1,3-butadiene or 1,4-hexadiene or acetylenically unsaturated compounds, such as methyl acetylene.
Procedures for the synthesis of substituted fluorenes used to produce metallocene polymerization catalysts are influenced by specific features of the fluorene ligand. The direct electrophilic substitutions of fluorene occur predominantly at the 2- or 2,7-positions having the highest electron density. For example, 2,7-di-t-butylfluorene can be prepared from the reaction of fluorene with t-butyl chloride in the presence of AlCl3:
As another example, as disclosed in EP1138687, 3,6-di-t-butyl fluorene can be prepared by the reaction of 2,2′-diiodo-4,4′-di-t-butyldiphenylmethane with copper as shown in the following reaction:
This reaction, which occurs at a high temperature (230-250° C.), results in a mixture of products. When using this method, several purification steps are needed in order to obtain the pure 3,6-di-tert-butyl-fluorene.